U.S. patent application number 09/896286 was filed with the patent office on 2002-02-07 for high temperature sensor.
Invention is credited to Birkhofer, Thomas, Maunz, Werner, Moos, Ralf, Muller, Ralf, Muller, Willi, Plog, Carsten.
Application Number | 20020014107 09/896286 |
Document ID | / |
Family ID | 7647402 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014107 |
Kind Code |
A1 |
Moos, Ralf ; et al. |
February 7, 2002 |
High temperature sensor
Abstract
High temperature substance sensor, including a substrate (4), a
device (6) for raising and maintaining the temperature of the
sensor, and a layer like capacitor structure (38) with structure
sizes smaller than 50 .mu.m, upon which a functional layer (18) is
applied. In accordance with the invention the layer-like capacitor
structure (38) is produced by the following: application of a
complete or already pre-structured electrically conductive layer as
precursor of the capacitor structure (38) using a thick layer
technique, structuring the electrically conductive layer using a
photolithographic structuring process.
Inventors: |
Moos, Ralf;
(Friedrichshafen, DE) ; Birkhofer, Thomas;
(Immenstaad, DE) ; Maunz, Werner; (Markdorf,
DE) ; Muller, Ralf; (Deggenhansertal, DE) ;
Muller, Willi; (Salem, DE) ; Plog, Carsten;
(Markdorf, DE) |
Correspondence
Address: |
PENDORF & CUTLIFF
P.O. Box 20445
Tampa
FL
33622-0445
US
|
Family ID: |
7647402 |
Appl. No.: |
09/896286 |
Filed: |
June 29, 2001 |
Current U.S.
Class: |
73/31.05 |
Current CPC
Class: |
G01N 2027/222 20130101;
G01N 27/226 20130101 |
Class at
Publication: |
73/31.05 |
International
Class: |
G01N 009/00; G01N
007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2000 |
DE |
100 31 976.9-52 |
Claims
1. High temperature substance sensor, including a substrate (4), a
device (6) for raising and maintaining the temperature of the
sensor, and a layer-like capacitor structure (38) with structure
sizes smaller than 50 .mu.m, upon which a functional layer (18) is
applied, thereby characterized, that the layer-like capacitor
structure (38) is produced by the following: application of a
closed or already pre-structured electrically conductive layer as
precursor of the capacitor structure (38) using a thick layer
technique, structuring the electrically conductive layer using a
photolithographic structuring process.
2. High temperature substance sensor according to claim 1, thereby
characterized, that the structuring of the electrically conductive
layer is achieved according to the following: application of a
photosensitive resin layer upon the electrically conductive layer,
application of a photo mask, which corresponds to the capacitor
structure, upon the resin layer, exposure of the resin layer
covered with the photo mask to light, removal of the exposed areas
of the resin layer, removal of the areas of the electrically
conductive layer not covered by resin.
3. High temperature substance sensor according to claim 1, thereby
characterized, that the structuring of the electrically conductive
layer occurs in accordance with the following steps: application of
a photosensitive resin layer upon the electrically conductive
layer, application of a photo mask, which correspond to the
negative of the capacitor structure, upon the resin layer, exposure
of the photo mask covered resin layer to light, removal of the
unexposed areas of the resin layer, removal of the areas of the
electrically conductive layer not covered by resin.
4. High temperature substance sensor according to one of claims 1
through 3, thereby characterized, that the capacitor structure (38)
is an interdigitated capacitor structure.
5. High temperature substance sensor according to claim 4, thereby
characterized, that in the interdigitated capacitor structure, the
aspect ratio of the thickness of the electrode finger (d) to the
finger breadth (b) is greater than 0.10.
6. High temperature substance sensor according to one of the
preceding claims, thereby characterized, that the capacitor
structure (38) is comprised of a metal, in particular of gold or
platinum.
7. Use of a high temperature substance sensor according to one of
the preceding claims as gas sensor.
8. Use of a high temperature substance sensor according to one of
the preceding claims 1 through 6 as an exhaust gas sensor in the
exhaust gas of a vehicle with an internal combustion engine.
Description
[0001] The invention concerns a high temperature sensor,
particularly an exhaust gas sensor in the exhaust line of an
automobile.
[0002] In order to meet the ever more stringent governmental
requirements with respect to air quality, very selective gas
sensors are necessary. Such sensors can be employed for example for
monitoring pollutant levels, or to activate an alarm when a
threshold concentration of a dangerous or poisonous gas in the
environmental atmosphere has been exceeded. It is also possible to
employ such gas sensors directly in the exhaust gas of an internal
combustion process. Examples thereof include selective hydrocarbon
sensors such as known for example from EP 0 426 989 or selective
ammonia sensors as known for example from DE 197 03 796.
[0003] The mentioned examples concern gas sensors produced using
planar technology (and in particular thick layer technology or thin
layer technology). In FIG. 1 various views of the typical design of
such a sensor are schematically illustrated. A substrate 4 has
provided on the sensor lower side a structure 6 for heating and
eventually temperature measurement, and has provided on the sensor
upper side at the sensor tip a capacitor structure. This structure,
which is again shown in FIG. 2 in enlarged representation, is
comprised of a plurality of staggered or offset electrodes 8 which
are alternatingly connected to conductor line 10 or conductor line
12. The conductors 10 and 12 have respective contact pads 14 and 16
on the sensor connection side, onto which connector wires are
applied. If an alternating current is applied to the two
conductors, then the capacitates C.sub.L of this structure
(referred to in the following as empty capacity) can be measured.
Since this capacitor structure looks similar to inter-digitating
fingers, such a structure is referred to also as interdigitatec
capacitor (IDC). If now upon this IDC structure a functional layer
18--not shown for purposes of better understanding--is applied, of
which the electrical characteristic changes upon exposure to a gas,
then one can construct therewith a gas sensor. Such a construction
is in principle not only suitable for sensors which detect
components of a gas mixture, but rather also for all chemical or
substance sensors.
[0004] The term "substance sensor" is intended herein to mean a
sensor for determination of concentrations of a substance in a
substance mixture, that is, for example, a sensor for determining
the concentration of a component of a gas mixture or a sensor for
determining a component of a fluid or a sensor which changes its
output signal on the basis of an interaction with a gas or a
fluid.
[0005] The above described arrangement comprised of substrate,
heating and/or temperature measurement resistor device and IDC
structure will in the following be referred to as "U-carrier". A
sensor in this respect is also comprised of at least a transducer
and a functional layer.
[0006] Estimation of Signal Size
[0007] The signal change to be measured depends upon the geometry
of the IDC structure. This is shown again in FIG. 2 in enlarged
view. The entire IDC structure has as external dimensions the
length L and the breadth B. Across the breadth B electrode fingers
of the breadth b are provided in separation s. One can therewith
imagine the entire capacitor as a parallel circuit (electrically
switched in parallel) comprised of multiple component capacitors,
wherein each partial capacitor is comprised of two adjacent
fingers. The empty capacity of these partial capacitors, and
therewith also the total empty capacity C.sub.L, increases with the
finger length L. With a reduction in the finger separation s the
empty capacity of the partial capacitors likewise increases, since
the density of the field line or line of electric flux between two
fingers increases (in comparison: in plate capacitors the capacity
is inversely proportional to plate separation). Since the total
capacity is based upon the parallel circuitry of the partial
capacities, the total capacity is the larger the greater the number
of partial capacitors which can be provided within the breadth B
with decreasing finger breadth b, thus the capacity of the total
capacitor increases, since the number of the parallel switched
partial capacitors increases with decreasing finger breadth b at
constant outer dimension B. With decreasing finger spacing s in
accordance therewith the capacity of the total condenser even
increases over-proportionally (almost quadratically), since on the
one hand the number of the partial condensers and on the other hand
their capacity increases.
[0008] The height of the electrode (layer thickness) is only of
minimal consequence.
[0009] In the following a few theoretical calculations of the total
capacity C.sub.L will be presented, which are carried out using a
finite element method. Therein the measurements of a typical IDC
structure, that is, approximately 5 mm.times.6 mm (L.times.B), is
used as basis. For the relative dielectric constant,
.epsilon..sub.r was presumed to have a value of
.epsilon..sub.r.apprxeq.10 as disclosed in published literature as
conventional for Al.sub.2O.sub.3 substrates. The results of the
calculations confirm that the layer thickness of the IDC structures
can be disregarded.
[0010] It has further been determined, as best seen in FIG. 3, that
an optimal relationship of line separation s and finger breadth b
of s/b.apprxeq.2 exists, at which the total empty capacity C.sub.L
reaches a maximum. In FIG. 3 a finger separation of s=20 .mu.m was
presumed. At a finger breadth of b=9.88 .mu.m there is the maximum
empty capacity. If one varies the finger separation s, then one can
determine that the value of the optimal relationship is almost
independent of the separation of the fingers. One achieves for
example at s=20 .mu.m an optimal value for the finger breadth of
b=9.88 .mu.m (s/b=2.024) and at s=10 .mu.m an optimal finger
breadth of b=0.54 .mu.m (s/b=1.203).
[0011] The optimal empty capacity for finger separations ranging
from 10 .mu.m to 30 .mu.m is shown in FIG. 4. One can recognize
that at a finger separation of approximately 20 .mu.m a total empty
capacity C.sub.L of almost 40 pF can be achieved. Table 1 clearly
shows the relationship between the geometric size b and s and the
total empty capacity C.sub.L. At structure breadths for s and b of
approximately 100 .mu.m one achieves only a total empty capacity of
C.sub.L<10 pF.
1TABLE 1 Finger Optimal Finger Separation Separation Total Empty
Maximal Capacity s/.mu.m b/.mu.m Capacity C.sub.L/pF Change
.DELTA.C.sub.max/pF 10 4.54 82.33 4.12 15 7.22 53.22 2.66 20 9.88
39.28 1.96 25 12.53 31.10 1.56 30 15.15 25.73 1.29
[0012] If one next applies the functional layer 18, then the
measurable capacity increases, depending upon the dielectric
constant .epsilon..sub.r of the functional layer and its thickness.
It can however be shown that the influence of the layer thickness
of the functional layer in particular at values of the dielectric
constant .epsilon..sub.r<5 hardly plays any roll. If one
presumes that the supplemental capacity, which is attributable to
the functional layer, corresponds to the half value of the empty
capacity, and if one further presumes that the supplemental
capacity during gas sampling changes at a maximal of 10% of its
value, then one obtains the maximal capacity change
.DELTA.C.sub.max to be measured, which is entered in the fourth
column of Table 1. It is immediately evident from Table 1 that one,
in order to even be able to make reliable measurements, must have
as small as possible finger breadth b and finger separation s. This
is in particular then the case, when long conductors or lead lines,
which conventionally exhibit capacities of a few pF/m, are
required. This is for example the case, when the sensor is to be
employed in the exhaust gas stream of an automobile, in order to be
able to measure the ammonia or hydrocarbon content in the exhaust
gas of an automobile. Therein it is to be observed, that even this
lead line or conductor capacity is conventionally not constant, but
rather changes with the environmental temperature. This conductor
capacity can only be compensated for in complex or expensive
manner.
[0013] Further complicating matters is that small measurement
currents are used. Thus one calculates at an alternating voltage
amplitude of 1V and a capacity of 50 pF at a measurement frequency
of 1 kHz a capacitive current of 314 nA, wherein the maximal signal
change (that is, the measurement effect) however only corresponds
to approximately 16 nA. If one wants to resolve the sensor signal
to 1%, then a measuring current of 160 pA must be resolved. Since
the measuring current in a capacitive system with constant applied
measurement voltage amplitude increases with increasing frequency,
then one should measure at higher frequencies, which however may
bring about a danger of intensified stray effect and
electromagnetic interference. Since with a given measurement
voltage the measurement current is proportional to the capacity,
this is a further reason to select as fine as possible structures,
that is, high capacity for the IDC structure.
[0014] The above discussed range of problems for functional layers,
of which the capacitive characteristics change upon exposure to or
interaction with gas, applies in appropriate manner also for
sensors of which complex impedance (complex alternating current
resistance) changes with gas sampling. Above all, high ohm
functional layers, which provide only small capacitive values,
require a fine as possible structure.
[0015] As a structure breadth which provides signals which are just
barely detectable with economically justifiable measurement
technology and subsequently electrically processable, 50 .mu.m has
been found to be satisfactory.
[0016] Planar gas sensors can be produced either in accordance with
the thick layer technique or the thin layer technique (typically
processes of the thin layer technique: sputtering, vapor depositing
or CVD). Examples, in which also the processes are disclosed, which
an be used for production of substance sensors in the thick layer
technology, can be found in J. Gerblinger, M. Hausner, H. Meixner:
Electric and Kinetic Properties of Screen-Printed Strontium
Titanate Films at High Temperatures, J. Am. Cer. Soc., 78[6]
1451-1456 (1995) or M. Prudenziati (Editor): Thick Film Sensors,
Particularly Section I: Thick Film Technology, pages 3-37,
Elsevier-Verlag, 1994 or in DE 37 23 052. It is possible to combine
thin layer techniques and thick layer techniques (so called hybrid
technology), but this is expensive.
[0017] In the manufacturing of high temperature substance sensors
the following requirements are to be taken into consideration (the
term high temperature sensors is understood to mean those sensors
which are heated to temperatures above 300.degree. C. This type of
requirement is placed particularly upon exhaust gas sensors, for
example in the exhaust gas of internal combustion engines in
vehicles):
[0018] On the one hand, thin layer techniques make it possible to
produce the finest structure breadths of as small as a few .mu.m,
which for the above mentioned example would be quite sufficient.
However, thin layer processes only make possible layer thicknesses
below 1 .mu.m. In rough to abrasive environmental conditions, in
particular with long operation at high temperatures, such thin
layers are not sufficiently durable over time. Further, it is
necessary, when using conventional high temperature stable
electrode materials such as gold or platinum for the thin layers,
so called adhesion promoters which for example could be a few nm
thick layers of chrome or titanium. At the high temperatures at
which high temperature gas sensors operate, for example exhaust gas
sensors, these materials diffuse to the upper surface of the
electrode and there react with the functional layer 18. This
changes the functional layer, and the sensor can become
desensitized to the gas to be detected. Besides this many
functional layers, in particular zeolites or complexes of
multi-oxides cannot be produced in the thin layer technique.
Besides this one requires for the production of components in the
thin layer technology normally specific or particular substrates
with a very low surface roughness, which is substantially more
expensive (by a factor of 5 to 10) than conventional ceramic
substrates. Since most thin layer processes are vacuum processes,
one requires for the thin layer techniques complex and expensive
apparatus, which can generally be amortized only when producing
large patches of pieces.
[0019] The above discussed arguments lead to the conclusion, that
the thick layer technique would be the most suitable manufacturing
process for high temperature gas sensors both for technical as well
as cost reasons.
[0020] However, unfortunately, using the thick layer technique
conventionally the finest structure breadths that can be
reproducibly produced are only in the range of 70 .mu.m to 100
.mu.m. The required resolution of below 50 .mu.m, in particular
approximately 20 .mu.m, could not be achieved with the conventional
thick layer techniques for gas sensors according to the state of
the art.
[0021] It is thus the task of the invention to provide a high
temperature substance sensor with structural sizes smaller than 50
.mu.m, with which the described range of problems with respect to
the manufacture of the sensor can be overcome.
[0022] This task is solved by the high temperature substance sensor
according to Patent claim 1. Advantageous embodiments of the
invention are the subject of further claims.
[0023] In accordance with the invention the production of the
capacitor or capacitor structure of the high temperature substance
sensor occurs from a combination of the thick layer technique
process and a photolithographic structuring process, which is
employed in the planar technique for production of semiconductor
components. It is now for the first time employed in the
manufacture of substance sensors. The production of the other
layers of the high temperature substance sensors occurs
advantageously using the thick layer technique, for example with
the silkscreen printing or stencil printing technique.
[0024] For the production of the capacitor structure, there is
first produced, using the thick layer technique, a complete
(closed) or already pre-structured capacitor layer as a precursor
of the capacitor structure. Subsequently there occurs the
structuring of the capacitor layer using photolithography.
[0025] The inventive high temperature substance sensor is
particularly suitable for employment has exhaust gas sensor in
internal combustion exhausts, for example in the exhaust of an
automobile.
[0026] It can be constructed for example as an ammonia or
hydrocarbon sensor.
[0027] An example of the invention will be described with reference
to Figures. There is shown:
[0028] FIG. 1 the design of a substance sensor, with various
views;
[0029] FIG. 2 the capacitor structure of a substance sensor;
[0030] FIG. 3 the capacity C of an interdigitated capacitor
structure depending upon the finger separation b with constant line
separation s;
[0031] FIG. 4 the maximal capacity C of an interdigitated capacitor
structure with variable finger separation b and constant line
separation s for multiple values of the line separation s;
[0032] FIGS. 5,6 respectively diagrams illustrating the sequence
for the manufacture of an inventive high temperature substance
sensor;
[0033] FIG. 7 measurement protocol, obtained using a substance
sensor produced using a hybrid technique, of which the finger
breadth s=10 .mu.m;
[0034] FIG. 8 measurement protocol, obtained with a substance
sensor produced using the thick layer technique, of which the
finger breadth s=100 .mu.m;
[0035] FIG. 9 measurement protocol, obtained using a substance
sensor according to the invention, of which the finger breadth s=20
.mu.m.
[0036] The production of the inventive high temperature sensor will
be described step-by-step for a typical example and with reference
to FIGS. 5 and 6.
[0037] Step 1:
[0038] Upon a ceramic substrate 4, which is comprised for example
of a conventional 96% Al.sub.2O.sub.3, there is applied on the
lower side a structured heater and temperature measurement resistor
structure 6, which can be comprised for example of platinum, and
this is subsequently fired at 1400.degree. C. The application of
the heater and temperature measuring resistor structure 6 occurs
using a silkscreen printing technique, as an example of a thick
layer technique process.
[0039] Step 2:
[0040] Upon this layer a ceramic cover layer 32, for protecting the
heat and temperature measurement resistor structure 6, is applied
over the entire surface using silkscreen printing and fired at, for
example, 1300.degree. C.
[0041] Step 3:
[0042] Next, on the other side of the substrate a barrier layer 34,
which can be comprised for example of platinum, having an
appropriate structure is applied using silkscreen printing and
fired at 1250.degree. C.
[0043] Step 4:
[0044] Upon the barrier layer 34 there is likewise applied using
silkscreen printing a ceramic layer, glass layer or a glass ceramic
layer 36 for electrical insulation, and this is fired.
[0045] Steps 3 and 4 are only necessary when the gas sensor
requires a layer for electrical insulation or sheilding. This
serves to shield the sensor measuring process against interferences
on the basis of the heat process at the heat and temperature
measurement resistor layer 6.
[0046] Step 5a:
[0047] Next, by means of silkscreen printing, a gold layer 38 is
applied either over the entire surface or already pre-structured,
and fired.
[0048] Step 5b:
[0049] Upon this gold layer 38 a photosensitive resin layer is
applied by means of a spin coat process and is so heated, that the
resin cross-links. A photo mask, which contains the IDC structure,
is placed precisely upon the photo resin layer and the photo resin
is illuminated or exposed to radiation. Subsequently it is
developed, whereby the illuminated parts of the resin can be
removed in a suitable alkaline solvent. The resin part now
remaining upon the gold layer 38 is an image of the IDC structure.
In an etch bath, comprised for example of an
iodide-potassium-iodide solution, the surfaces of the gold layer 38
not covered by the resin are removed. Subsequently carefully the
rest of the etching solution must be removed using distilled water.
Then in a suitable solvent (for example acetone) the remainder of
the resin layer is removed. Thereunder now the IDC structure
becomes visible, and once again is cleansed. In order to remove any
possible present resin or solvent residues, the gold layer is once
again fired for cleansing. It is also possible to bypass the step
of removing the resin layer using the solvent by proceeding
directly to the step of firing the resin. According to this process
the IDC structure is produced and now the functional layer can be
applied. The structure is sketched in FIG. 6. The maximal
achievable resolution was, in the framework of experiments,
dependent upon the selection of the gold paste, determined to be
approximately 15 .mu.m. The work should be carried out in a clean
room, since impurities can result immediately in a defect (short
circuit or interruption) in the IDC structure. The employed gold
paste should be so prepared or produced, that in the fired
condition a flat as possible surface if produced, upon which the
illumination or photo mask can be laid.
[0050] Step 5 is a combination of a process of the typical thick
layer technique with a photolithographic process as employed in the
planar technology for the manufacture of semiconductor components.
It is now used for the first time for the manufacture of gas
sensors. One obtains an IDC structure which exhibits all of the
required characteristics for the production of high temperature gas
sensors, such as layer thickness in the .mu.m-range, temperature
stability, and manufacturability on economical substrates as
conventionally employed in the thick layer technology. In addition,
such a transducer however also exhibits the above-described
essential fine resolution. In the above Step 5 the manufacture of
an IDC structure using a photolithographic structured gold layer is
described. Such an IDC structure can be produced using platinum or
other high temperature stable metals. In the case of platinum as
the work material for the IDC, a suitable platinum layer is applied
in the thick layer technology and this is structured using a
suitable resin and a suitable solvent.
[0051] Alternatively to the described photolithographic structuring
process, in which the applied photo mask corresponds to the
capacitor structure, and in which in a further step the illuminated
or irradiated area of the resin layer can be removed, also a
process can be employed using the so-called negative resin. Therein
the applied photo mask corresponds to the negative of the capacitor
structure, wherein in a further step the non-irradiated area of the
resin layer is removed.
[0052] Step 6:
[0053] Upon the completed transducers conductive strips 10 and 12
are printed using the silk printing technique and these are again
fired in. The contact pads 14 and 16 can be thickened once again
using for example silkscreen printing and firing of a suitable
paste, so that they can make good contact with the connecting
wires. The Step 6 is not shown in FIG. 6 for reasons of easier
understanding of the figures.
[0054] Step 7:
[0055] It could in certain cases be advantageous to print upon the
lead lines a protective layer and to subsequently fire the
protective layer. With Step 6 or Step 7 the transducer is
completed. Step 7 is omitted from FIG. 6 for easier overview.
[0056] Step 8:
[0057] On the transducer there is now applied the functional layer
18 likewise using the thick layer technique and fired.
[0058] The advantages of this inventive construction are once again
elucidated on the basis of the following example. As example for a
typical functional layer, a zeolite layer is employed which can be
used for a selective ammonia sensor for application in the exhaust
gas flow of an automobile.
[0059] FIG. 7 through FIG. 9 shows a measurement protocol, which
was obtained using various sensors. The sensors differ essentially
in the structure breadth. The functional layer of all three sensors
was prepared from the same zeolite batch according to the same
manufacturing process. Illustrated is the sequence of the signal
processor in a working temperature of the sensor upon exposure of
the sensor to 5 ppm, 10 ppm, 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100
ppm ammonia in the atmosphere, which simulate the exhaust gas of an
automobile. The measurement signals were recorded using an
impedance measurement bridge using respectively the same frequency
and evaluated as parallel circuit of a capacitor and a resistor
(dissipative capacitor).
[0060] The sensor A (FIG. 7) exhibits a finger breadth of s=10
.mu.m. Sensor A was produced using hybrid technology. The IDC
structure of sensor A was produced using the thin layer technique
and the zeolite layer was produced using the thick layer
technique.
[0061] Sensor B (FIG. 8) exhibited a finger breadth of s=100 .mu.m.
Sensor B was completely produced using the thick layer
technique.
[0062] Sensor C (FIG. 9) exhibited a finger breadth of s=20 .mu.m.
Sensor C was completely produced in accordance with the invention
following Steps 1 through 7.
[0063] It is readily apparent that the impedance of the Sensor A
produced using expensive hybrid technology at 0 ppm ammonia lies at
approximately 30 k.OMEGA., with changes of the impedance upon
exposure to gas of 10 k.OMEGA.. The capacity changed by
approximately 1.5 pF at zero values by 83 pF. Such values of
impedance in capacity are not economically measurable using
conventional technology.
[0064] The impedance of the Sensor B, which is produced using the
conventional thick layer technique, likewise exhibits changes about
a factor of 1.5. However the zero impedance is approximately 550
k.OMEGA. and is only measurable by a high ohm impedance analyzer.
The capacity also can only be determined by expensive or complex
means. Economical measurements cannot be carried out with the aid
of this sensor type.
[0065] Sensor C produced in accordance with the invention exhibits
an impedance change of approximately 30 k.OMEGA., with a zero
impedance value of 60 k.OMEGA.. The capacity changes by
approximately 2 pF at a zero impedance of 30.5 pF. Such values of
impedance in capacity are economically measurable with conventional
technology.
[0066] In addition, in accordance with the product produced in
accordance with the invention, a presently not understood effect
can be observed which leads to a heightened sensitivity. While in
the manufacture using a "pure" technique (thin layer technique or
thick layer technique, Sensor A or B) the relationship of the
impedance value of 0 ppm and 100 ppm ammonia corresponds to only
about 1.5, one observes with the inventive sensor an impedance
relationship of 2. In addition the relative capacity change
.DELTA.C/C.sub.0 to be measured is greatest using the inventive
Sensor type C at 6.3%. A possible explanation could lie in a
further advantage of this process which by the improved aspect
relationship A, that is in enlarged ratio or relationship of the
thickness d of the electrode finger to the finger breadth b
(A.sub.v=d/b), increases the field lines in the functional layer.
For comparison: with Sensor A the aspect relationship A.sub.v=0.2
.mu.m/10 .mu.m=2%. With Sensor B the aspect relationship was
A.sub.v=8 .mu.m/100 .mu.m=8%. With Sensor C the aspect relationship
was A.sub.v=4 .mu.m/20 .mu.m=20%.
[0067] The described process can be employed for production of
transducers for substance sensors with any of various functional
layers. Its advantages are demonstrated above all when high ohm or
capacitive functional layers are employed and produced using the
thick layer technique. Thereby it offers the benefit of the
temperature stability of sensors which are produced in the thick
layer technique, in combination with the structural resolution of
sensors which are produced in accordance with the thin layer
technique. Further, aspect relations can be achieved which retain
the electrical field lines more in the functional layer. Therewith
the sensor signal in relationship to the zero impedance value is
larger.
* * * * *